Precursor compound of radioactive halogen-labeled organic compound

ABSTRACT

It is intended to provide a novel amino acid organic compound which can be used as a labeling precursor compound for radioactive halogen-labeled amino acid compounds including [ 18 F]FACBC, and which prevents methanol from remaining in the radioactive halogen-labeled amino acid compounds produced therefrom. The novel amino acid organic compound is a compound represented by the following formula: 
                         
wherein n is an integer of 0 or of 1 to 4; R 1  is an ethyl, 1-propyl or isopropyl substituent; X is a halogen substituent or a group represented by —OR 2 ; R 2  is a straight-chain or branched-chain haloalkylsulfonic acid substituent with one to 10 carbon atoms, trialkylstannyl substituent with 3 to 12 carbon atoms, fluorosulfonic acid substituent or aromatic sulfonic acid substituent; and R 3  is a protective group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.14/872,305, filed Oct. 1, 2015, which is a continuation of U.S.application Ser. No. 14/246,594, filed Apr. 7, 2014 (now U.S. Pat. No.9,381,259, issued Jul. 5, 2016), which is a continuation of U.S.application Ser. No. 12/085,679, filed May 29, 2008 (now U.S. Pat. No.8,758,724B2, issued Jun. 24, 2014), which is the U.S. National Phase ofInternational Application PCT/JP2006/323659, filed Nov. 28, 2006, andclaims the benefit of foreign priority under 35 U.S.C. § 119 based on JP2005-343653, filed Nov. 29, 2005, the entire disclosures of whichapplications are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a precursor compound which can besuitably used for production of radioactive halogen-labeled organiccompounds or active ingredients for diagnostic agents used in positronemission tomography and single photon emission computed tomography.

BACKGROUND ART

Nuclear medicine examination represented by positron emission tomography(hereinafter referred to as PET) and single photon emission computedtomography (hereinafter referred to as SPECT), is effective indiagnosing a variety of diseases including heart disease and cancer.These techniques involve administering an agent labeled with a specificradioisotope (hereinafter referred to as radiopharmaceutical) to apatient, followed by detecting γ-rays emitted directly or indirectlyfrom the agent. Nuclear medicine examination is characteristic in thatit has not only high specificity and sensitivity to diseases, but alsoan advantage of providing information on the functionality of lesions,compared to other examination techniques.

For example, [¹⁸F]2-fluoro-2-deoxy-D-glucose (hereinafter referred to as“¹⁸F-FDG”), one of radiopharmaceuticals used for PET examination, tendsto be concentrated in area where glucose metabolism is enhanced, therebymaking it possible to specifically detect tumors in which glucosemetabolism is enhanced.

Nuclear medicine examination is performed by tracing a distribution ofan administered radiopharmaceutical, and data obtained therefrom varydepending on nature of the radiopharmaceutical. Thus, differentradiopharmaceuticals have been developed for different diseases, andsome of them are put into clinical use. There have been developed, forexample, various tumor diagnostic agents, bloodstream diagnostic agentsand receptor mapping agents.

In recent years, a series of radioactive halogen-labeled amino acidcompounds including [¹⁸F]1-amino-3-fluorocyclobutanecarboxylic acid(hereinafter referred to as [¹⁸F]FACBC) have been designed as novelradiopharmaceuticals, and their clinical application is underexamination (Patent Document 1, and non-Patent Documents 1 and 2).[¹⁸F]FACBC is considered to be effective as a diagnostic agent forhighly proliferative tumors, because it has a property of being taken upspecifically by amino acid transporter.

As processes for producing [¹⁸F]FACBC, there are disclosed processeswhich include: providing1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid methyl ester as a labeling precursor, substituting the triflategroup at position 3 of the precursor with radioactive fluorine, andcarrying out deprotection by subjecting the resulting compound to anacidic condition (Patent Document 1, and non-Patent Documents 1 and 2).

-   Patent Document 1: Japanese Patent Laid-open No. 2000-500442.-   Non-Patent Document 1: Jonathan McConathy et al., “Improved    synthesis of anti-[18F]FACBC: improved preparation of labeling    precursor and automated radiosynthesis.”, Applied Radiation and    Isotopes, (Netherlands), 2003, 58, p. 657-666.-   Non-Patent Document 2: Timothy M. Shoup et al., “Synthesis and    Evaluation of [18F]1-Amino-3-fluorocyclobutane-1-carboxylic Acid to    Image Brain Tumors.”, The Journal of Nuclear Medicine, 1999, 40, p.    331-338.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, investigations made by the present inventors have revealed thatthe processes for producing [¹⁸F]FACBC disclosed up until now allowmethanol to remain in the produced [¹⁸F]FACBC as a residual solvent.Methanol is specified as a class 2 solvent in ICH guideline “Impurities:Guideline for Residual Solvents” and treated as a solvent whose levelremaining in pharmaceuticals should be regulated.

The present invention has been made in light of the above describedcircumstances. Accordingly, an object of the present invention is toprovide a novel amino acid organic compound which can be used as alabeling precursor compound for radioactive halogen-labeled amino acidcompounds having a cyclobutane ring skeleton, including [¹⁸]FACBC, andwhich prevents methanol from remaining in the radioactivehalogen-labeled amino acid compounds produced therefrom.

Means for Solving the Problems

As a result of investigation, the present inventors have found that whenthe ester bound to the carbon atom at position 1 of the cyclobutane ringis formed with an alkyl with 2 or 3 carbon atoms, it is possible toprevent methanol from remaining in the synthesized compound. Thus, thepresent invention has been accomplished.

The present invention provides a precursor compound for radioactivehalogen-labeled organic compounds, which is represented by the followingformula (1):

In the above formula (1), n is an integer of 0 or of 1 to 4 anappropriate value of which may vary depending on kinds of radioactivehalogen-labeled amino acid compounds to be finally produced. Forexample, when the compound to be finally produced is a compound in whicha halogen is directly bound to the position 3 of the cyclobutane ring(e.g. [¹⁸F]FACBC), n is 0, while when the compound to be finallyproduced is a compound in which a halogen is bound to the position 3 ofthe cyclobutane ring via a methylene chain, such as[¹⁸F]1-amino-3-fuluoromethylcyclobutanecarboxylic acid, n is 1.

In the above formula (1), R¹ represents an ethyl, 1-propyl or isopropylsubstituent, and preferably an ethyl substituent.

In the above formula (1), X represents a halogen substituent or a grouprepresented by —OR². R² is selected from the group consisting ofstraight-chain or branched-chain haloalkylsulfonic acid substituentswith one to 10 carbon atoms, trialkylstannyl substituents with 3 to 12carbon atoms, fluorosulfonic acid substituents and aromatic sulfonicacid substituents, and is preferably a substitutent selected from thegroup consisting of toluenesulfonic acid substituent,nitrobenzenesulfonic acid substituent, benzenesulfonic acid substituent,trifluoromethanesulfonic acid substituent, fluorosulfonic acidsubstituent, perfluoroalkylsulfonic acid substituent, trimethylstannylsubstituent and triethylstannyl substituent. As a halogen substituent, abromo or chloro substituent can be preferably used.

R³ is selected from the group consisting of straight-chain orbranched-chain alkyloxycarbonyl substituents with 2 to 7 carbon atoms,straight-chain or branched-chain alkenyloxycarbonyl substituents with 3to 7 carbon atoms, benzyloxycarbonyl substituents having 7 to 12 carbonatoms which may be modified with a substitutent, alkyldithiooxycarbonylsubstituents with 2 to 7 carbon atoms, straight-chain or branched-chainalkylamide substituents with one to 6 carbon atoms, straight-chain orbranched-chain alkenylamide substituents with 2 to 6 carbon atoms,benzamide substituents with 6 to 11 carbon atoms which may be modifiedwith a substituent, cyclic imide substituents with 4 to 10 carbon atoms,aromatic imine substituents with 6 to 11 carbon atoms which may have asubstituent, straight-chain or branched-chain alkylamine substituentswith one to 6 carbon atoms, straight-chain or branched-chainalkenylamine substituents with 2 to 6 carbon atoms, and benzylaminesubstituents with 6 to 11 carbon atoms which may have a substituent.Preferably R³ is a substituent selected from the group consisting oft-butoxycarbonyl group, allyloxycarbonyl group, phthalimide group andN-benzylideneamine substituent, more preferably R³ is t-butoxycarbonylgroup or phthalimide group.

Effects of the Invention

The compound according to the present invention can be used as alabeling precursor compound for radioactive halogen-labeled amino acidcompounds having a cyclobutane ring skeleton. By use of the compoundaccording to the present invention as a labeling precursor, it has beenmade possible to prevent methanol from remaining in the producedradioactive halogen-labeled amino acid compounds.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a process for producing a compound of the present invention willbe described taking, as an example, synthesis ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester shown in FIGS. 1 to 3.

First, a solution of syn-5-(3-benzyloxycyclobutane)hydantoin in asaturated barium hydroxide solution is refluxed, and sulfuric acid isadded to the refluxed solution to adjust the pH of the same to about 7.The solution is then filtered and the filtrate is concentrated to allowsyn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid to precipitate aswhite crystals. The acid used for the pH adjustment may be an acid otherthan sulfuric acid, but it needs to be an acid that forms awater-insoluble inorganic salt with barium (FIG. 1, Step 1).

The syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid is fully driedto remove water and then dissolved in ethanol. A base and thionylchloride are then added to the ethanol solution in this order, stirredat room temperature, and then heated under reflux at about 95° C. Afterthe reaction has fully progressed, the solution is concentrated underreduced pressure to yieldsyn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid ethyl ester aswhite crystals (FIG. 1, Step 2).

The base added to the reaction solution in the above step may be anybase, as long as it can trap the hydrochloric acid produced during thereaction. Preferably triethylamine can be used. The amount of the baseto be used is the same as or larger than that of thionyl chloride.

The amount of thionyl chloride needs to be the same as or larger thanthat of the reaction raw material, namely,syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid. If the amount ofthionyl chloride is too small, it unfavorably occurs that ethylesterification does not progress sufficiently. If the amount of thionylchloride is too large, excess hydrochloric acid is produced, and thus alarger amount of base is unfavorably required. In preferred embodiments,the amount of thionyl chloride is equal to or smaller than 5 equivalentsof syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid.

Then, syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid ethyl esteris added to a solution of a small amount of base in an alcohol solventsuch as ethanol. The resultant suspension is stirred under cooling, andt-butyl dicarbonate is added to the suspension to allow them to react atroom temperature (FIG. 1, Step 3). As the alcohol solvent, ethanol canbe preferably used, though various kinds of alcohol can be used. Theamount of the base is required to be sufficiently small relative to thatof the alcohol, but if the amount is too small, the progress of thereaction becomes slow unfavorably. In preferred embodiments, a solutionin which the ratio of alcohol to base is 9:1 is used. The amount oft-butyl dicarbonate needs to be one equivalent or more ofsyn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid, and is preferably1.5 equivalents of syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid.

This operation makes it possible to yieldsyn-1-(N-(t-butoxycarbonyl)amino)-3-benzyloxy-cyclobutane-1-carboxylicacid ethyl ester.

Thesyn-1-(N-(t-butoxycarbonyl)amino)-3-benzyloxy-cyclobutane-1-carboxylicacid ethyl ester synthesized as above is dissolved in an alcohol solventsuch as ethanol or an acetate ester solvent such as ethyl acetate ester,and palladium-on-activated carbon (amount: 10 w/w % or more relative tothe substrate) is added to the solution in an atmosphere of hydrogen toallow them to react under stirring at room temperature. The reactionsolution is then filtered through Celite, and the filtrate isconcentrated and purified to yieldsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester (FIG. 2, Step 4).

The resultantsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester is dissolved in a base such as pyridine, followed byaddition of trifluoromethanesulfonic anhydride. A target compound,syn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester is yielded by adding water, an organic solvent such asether, and acid to the resultant solution and purifying the organiclayer (FIG. 3, Step 5).

Compounds of the present invention other than the above described onecan also be synthesized through the steps similar to those describedabove. For example, when a compound is synthesized in which ahaloalkylsulfonic acid ester substituent other than the triflatesubstituent, an alkylsulfonic acid ester substituent or an aromaticsulfonic acid ester substituent is bound to the carbon atom at position3 of the cyclobutane ring, the reaction in the step 5 can be carried outin the same manner as above, except that a different halogen sulfonyl orsulfonic anhydride is used instead of trifluoromethanesulfonicanhydride.

When a compound is synthesized in which a trialkylstannyl substituent isbound to the carbon atom at position 3 of the cyclobutane ring, analcohol compound ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester or the like is oxidized into a ketone or aldehydecompound, and the ketone or aldehyde compound is subjected to Wittigreaction using a phosphonium salt such as phosphonium iodomethylene toform a vinyl halide at position 3, followed by the reaction with atrialkyltin hydride. A compound in which a halogen is bound to thecarbon atom at position 3 can be obtained by allowing the abovedescribed alcohol compound to react with a hydrogen halide or the like.

When a compound is synthesized in which an alkyloxycarbonyl substituentother than a t-butoxycarbonyl substituent, an alkenyloxycarbonylsubstituent or a benzyloxycarbonyl substituent is bound to the aminogroup at position 1, the reaction in the above described step 3 can beperformed using alkylchloroformates, alkenylchloroformates orbenzylchloroformates respectively, instead of t-butyl dicarbonate.Similarly, when a compound is synthesized in which a cyclic imidesubstituent is bound to the amino group, various cyclic acid anhydridessuch as phthalic anhydride can be used for the reaction with the aminogroup in the above described step 3. A compound in which an aromaticimine substituent is bound to the amino group can be synthesized byallowing benzaldehyde having a substituent to react with the amino groupin the step 3. Compounds having other functional groups can also besynthesized using known methods in combination (Theodora W. Greene,“Protective groups in organic synthesis”, 3^(rd) edition, USA, Jon Wiley& Sons, Inc., 1999, pp. 531, 550-561, and 573-586).

When a 1-propylester form and isopropylester form are synthesized,1-propanol and isopropanol may be used, respectively, as the alcohol forthe reaction in the above step 2.

Next, as an example of use of the novel amino acid organic compoundsaccording to the present invention, a method will be described in whichanti-[¹⁸F]FACBC is synthesized using the above synthesizedsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester.

The synthesis of anti-[¹⁸F]FACBC is carried out in two steps: a step ofadding radioactive fluorine to the precursor; and a step of deprotectingthe compound to which radioactive fluoride has been added.

Radioactive fluorine can be obtained by a known method, for example, amethod in which H₂ ¹⁸O enriched water is used as a target and exposed toproton bombardment. In this instance, radioactive fluorine exists in theH₂ ¹⁸O enriched water used as a target. The H₂ ¹⁸O enriched watercontaining radioactive fluorine is allowed to pass through, for example,an anion-exchange column so that the radioactive fluorine is adsorbedand collected on the column, thereby being separated from the H₂ ¹⁸Oenriched water. Thereafter, a potassium carbonate solution is allowed topass through the column to elute the radioactive fluorine, and theeluate is supplemented with a phase transfer catalyst and is evaporatedto dryness, thereby activating the radioactive fluorine.

Then, the dried radioactive fluorine is dissolved in acetonitrile, andthesyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester, as a precursor, is added to the acetonitrile solutionto allow them to react under heating. As a result, radioactive fluorineis added to the precursor, wherebyanti-[¹⁸F]1-(N-(t-butoxycarbonyl)amino)-3-fluorocyclobutane-1-carboxylicacid ethyl ester is synthesized.

The resultantanti-[¹⁸F]1-(N-(t-butoxycarbonyl)amino)-3-fluorocyclobutane-1-carboxylicacid ethyl ester is deprotected to yield anti-[¹⁸F]FACBC as a targetcompound. The deprotection can be performed, for example, by providingan acidic condition. The acidic condition can be provided by variousmethods, for example, a method in which an acid is added to a solutionthat containsanti-[¹⁸F]1-(N-(t-butoxycarbonyl)amino)-3-fluorocyclobutane-1-carboxylicacid ethyl ester. The amount of the acid to be added need not berestricted as long as the amount can provide an acidic conditionsufficient for the deprotection.

The other compounds of the present invention other than the abovedescribed compound can also be used as labeling precursors ofradioactive halogen-labeled compounds in the manner similar to thatdescribed above.

For example, compounds in which a trialkylstannyl substituent is boundto the carbon atom at position 3 of the cyclobutane ring can be mixedand reacted with various radioactive halogens and oxidizers depending onthe objective so as to yield radioactive halogen-labeled compounds.Compounds in which a halogen substituent is bound to the carbon atom atthe position 3 can be labeled with a radioactive halogen usingnucleophilic displacement reaction or isotopic exchange reaction. Whenlabeling with a radioactive halogen is performed using nucleophilicdisplacement reaction, the following displacement reaction can beperformed. For example, the halogen bound to the carbon atom at position3 is iodine, the iodine can be displaced by fluorine, chlorine orbromine, when the halogen bound to the carbon atom at position 3 isbromine, the bromine can be displaced by chlorine or fluorine, and whenthe halogen bound to the carbon atom at position 3 is chlorine, thechlorine can be displaced by fluorine.

EXAMPLES

The present invention will be now described in further detail withreference to Examples; however, it should be understood that the detailsof the Examples are not intended to limit the present invention.

The analytical conditions under which gas chromatography was carried outin each Example and Comparative Example were as follows.

Apparatus: GC-1700AF/aoc (manufactured by Shimadzu Corporation)

Column: SPB-1 (manufactured by SUPELCO, 30 m×0.53 mm I.D., particle sizeof packing: 3 μm)

Column temperature: 40° C. (3.3 minutes)→90° C. (0.5 minutes)(temperature increase rate: 20° C./min)

Inlet temperature: 250° C.

Detector temperature: 220° C.

Carrier gas: helium

Split ratio: 1:10

Linear velocity: 30 cm/sec

Example 1 Synthesis ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester

Hydrolysis of Syn-Hydantoin (FIG. 1, Step 1)

Syn-5-(3-benzyloxycyclobutane)hydantoin was synthesized in accordancewith the method described in a literature (Jonathan McConathy et al.,Applied Radiation and Isotopes, 2003, 58, p. 657-666).

A solution of 72.8 g (corresponding to 0.418 mol) of3-benzyloxycyclobutane-1-one in 2.86 L of ethanol was added dropwise toa solution prepared by dissolving 397 g (corresponding to 4.13 mol) ofammonium carbonate and 88.4 g (corresponding to 1.65 mol) of ammoniumchloride in 2.86 L of water, and stirred at room temperature for 30minutes. Then, 121.0 g (corresponding to 1.86 moles) of potassiumcyanide was added to the mixture and stirred at 60° C. overnight. Thereaction solution was concentrated, and the resultant yellow solid waswashed with 1.06 L of water to remove salts. The solid was subjected toazeotropic distillation with 927 mL of methanol and purified by silicagel column chromatography (elution solvent:dichloromethane/methanol=98/2) to yield 55.3 g ofsyn-5-(3-benzyloxycyclobutane)hydantoin.

250 mL of saturated barium hydroxide solution was added to 6.15 g(corresponding to 25 mmol) of syn-5-(3-benzyloxycyclobutane)hydantoinand refluxed under heating in an oil bath at 114° C. for 24 hours orlonger. Then, TLC analysis was performed using, as mobile solvents, twokinds of systems: chloroform/methanol=5/1 (Rf value ofsyn-hydantoin=around 0.6) and chloroform/methanol=95/1 (Rf value ofsyn-hydantoin=around 0.3), and the completion of the reaction wasconfirmed (by coloration with UV and phosphomolybdic acid).

After the completion of the reaction is confirmed, the reaction solutionwas cooled to room temperature, and about 24 mL of 1 mol/mL sulfuricacid was added to neutralize the reaction solution. After theneutralization, the reaction solution was further stirred at roomtemperature for 5 minutes, and the formed precipitate was removed byfiltration. The filtrate was concentrated to yield 5.67 g ofsyn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid as white crystals.

Ethyl Esterification (FIG. 1, Step 2)

5.67 g of syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid, whichhad been fully dried to remove water, was dissolved in 200 mL ofethanol. To this solution, 9.5 mL (corresponding to 75 mmol) oftriethylamine was added and cooled at −78° C. for 20 minutes, and then4.6 mL (corresponding to 62.5 mmol) of thionyl chloride was added. Thereaction solution was stirred at 0° C. for 1 hour and at roomtemperature for 1 hour, followed by heating under reflux in an oil bathat 95° C. overnight. The completion of the reaction was confirmed by TLCanalysis using a mobile solvent of chloroform/methanol=95/1 (Rf value ofthe target compound=around 0.6) (confirmed by coloration with UV andphosphomolybdic acid). After the completion of the reaction isconfirmed, the reaction solution was concentrated under reduced pressureto yield 7.64 g of syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acidethyl ester as white crystals.

Addition of Boc (FIG. 1, Step 3)

7.64 g of syn-1-amino-3-benzyloxycyclobutane-1-carboxylic acid ethylester was dissolved in 250 mL of a mixed solution ofethanol/triethylamine=9/1. After the solution was cooled in an ice bathfor 15 minutes, 8.6 mL (corresponding to 37.5 mmol) of t-butyldicarbonate was added to the solution and stirred at room temperatureovernight. The completion of the reaction was confirmed by TLC analysisusing a mobile solvent of hexane/ethyl acetate=1:1 (Rf value of thetarget compound=around 0.6) (confirmed by coloration with UV andmolybdic acid). After the completion of the reaction was confirmed, thereaction solution was concentrated under reduced pressure to yield whitecrystals as a residue. To the residue, 150 mL of cooled ethyl acetateand 150 mL of 0.5 mol/L cooled hydrochloric acid were added, stirred inan ice bath for 5 minutes, and left to stand until separation occurred.The organic layer was extracted and washed with 150 mL of water twice,with 150 mL of a saturated aqueous solution of sodium hydrogencarbonate, with 150 mL of water twice and with 150 mL of saturatedsaline solution twice in this order, dried with anhydrous sodiumsulfate, and concentrated under reduced pressure to yield yellow oilymatter. Separately, the water layer was extracted and washed with 150 mLof ethyl acetate twice, with 150 mL of water twice and with 150 mL ofsaturated saline solution in this order, dried with sodium sulfateanhydride, and concentrated under reduced pressure to recover a smallamount of yellow oily matter. By these operations, 8.82 g of lightyellow oily matter was obtained. The residue was purified by silica gelcolumn chromatography (hexane/ethyl acetate=1/1) to yield 8.04 g(corresponding to 23 mmol) ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-benzyloxy-cyclobutane-1-carboxylicacid ethyl ester as white crystals.

Debenzylation (FIG. 2, Step 4)

To 8.04 g (corresponding to 23 mmol) ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-benzyloxy-cyclobutane-1-carboxylicacid ethyl ester, was added 150 mL of ethanol and then 960 mg ofpalladium-on-activated carbon (10% palladium) to perform replacementwith hydrogen under stirring at room temperature overnight. After thereaction, palladium-on-activated carbon was removed by filtration usingCelite, and the filtrate was concentrated under reduced pressure toyield 5.74 g of white crystals as a residue. The reaction was traced byTLC analysis using a mobile solvent of hexane/ethyl acetate=1/1 (Rfvalue of the target compound of reaction=around 0.2) (confirmed bycoloration with UV and ninhydrin) to confirm the completion of thereaction. Then, the residue was purified by silica gel columnchromatography (hexane/ethyl acetate=1/1, hexane/ethyl acetate=4/1) toyield 5.36 g (corresponding to 20.7 mmol) ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester as white crystals.

Triflation (FIG. 3, Step 5)

2.07 g (8 mmol) ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester was dissolved in 26 mL of pyridine and stirred in anice bath for 20 minutes. Then, 2.0 mL (corresponding to 12 mmol) oftrifluoromethanesulfonic anhydride was added and stirred for 30 minutes.The reaction was traced by TLC analysis using a mobile solvent ofhexane/diethyl ether=1:1 (Rf value of the target compound ofreaction=around 0.6) (confirmed by coloration with ninhydrin) to confirmthe completion of the reaction. After confirming the completion of thereaction, 100 mL of water and 100 mL of ether were added to the reactionsolution, and extraction and washing was performed with 100 mL of 1mol/L hydrochloric acid twice, with 100 mL of water twice and with 100mL of saturated saline solution twice in this order. After drying withsodium sulfate anhydride, concentration under reduced pressure wasperformed to yield 2.78 g of light yellow crystals. The reaction mixturewas purified by silica gel chromatography (hexane/diethyl ether=3/1) toyield white crystals, and the resultant white crystals were againrecrystallized using pentane/diethyl ether to yield 1.84 g(corresponding to 4.7 mmol) ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester.

The NMR measurement results (internal standard: tetramethylsilane) ofthe obtainedsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester were as follows.

NMR apparatus used: JNM-ECP-500 (manufactured by JEOL, Ltd.)

¹H-NMR (solvent: CDCl₃, resonance frequency: 500 MHz): δ5.41-5.35 (m,1H), 5.32 (b, 1H), 4.26 (q, 2H, J=7 Hz), 3.10-3.02 (m, b, 4H), 1.45 (s,9H), 1.31 (t, 3H, J=7.0 Hz)

¹³C-NMR (solvent: CDCl₃, resonance frequency: 125 MHz): δ172.60, 154.46,118.48, 75.88, 51.97, 40.87, 28.29, 14.11

Comparative Example 1

Anti-[¹⁸F]FACBC was synthesized usingsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid methyl ester as a labeling precursor, and the measurement was madeof the residual solvent in the synthesized anti-[¹⁸F]FACBC.

Syn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid methyl ester was synthesized in accordance with a method describedin a literature (Jonathan McConathy et al., Applied Radiation andIsotopes, 2003, 58, p. 657-666).

[¹⁸F]fluoride ion-containing H₂ ¹⁸O (radioactivity: 3.27 GBq, acorrected value at the time of starting synthesis) was allowed to passthrough an anion-exchange column to adsorb and collect [¹⁸F]fluoride ionon the column. Then, a mixture of an aqueous solution of potassiumcarbonate (133 mmol/L, 0.3 mL) and a solution of 40 mg of Kryptfix 222(under trade name, manufactured by Merck & Co., Inc.) in 1.5 mL ofacetonitrile was allowed to pass through the same column to elute[¹⁸F]fluoride ion.

The eluate was heated to 110° C. to evaporate water, and was subjectedto azeotropic distillation with addition of acetonitrile (0.5 mL×2),followed by evaporation to dryness. To the dried [¹⁸F]fluoride, asolution of 30 mg of1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid methyl ester in 1 mL of acetonitrile was added and heated at 85° C.for 3 minutes. Then, 4 mL of diethyl ether was added to the solution andfurther 3 mL of the same was added twice, and the mixture was allowed topass through Sep-PakSilica (under trade name, manufactured by JapanWaters) to yield a solution of a [¹⁸F]fluorine-labeled compound inacetonitrile/diethyl ether.

To the obtained solution of the [¹⁸F]fluorine-labeled compound inacetonitrile/diethyl ether, 1.5 mL of 4 mol/L hydrochloric acid wasadded and heated at 120° C. for 15 minutes to perform deprotection toyield anti-[¹⁸F]FACBC. The obtained anti-[¹⁸F]FACBC was subjected to gaschromatography under the above described conditions to quantitativelydetermine methanol and ethanol. As shown in Table 1, methanol wasdetected at concentrations of 17.4±0.6 ppm.

TABLE 1 Quantitative analyses of methanol and ethanol Average StandardSolvent Content (ppm) (ppm) deviation Methanol 1 18.0 17.4 0.6 2 17.1 317.0 Ethanol 1 not detected 2 not detected 3 not detected

Example 2

[¹⁸F]fluoride ion-containing H₂ ¹⁸O (radioactivity: 36.63 GBq, acorrected value at the time of starting synthesis) was allowed to passthrough an anion-exchange column to adsorb and collect [¹⁸F]fluoride ionon the column. Then, a mixed solution of an aqueous solution ofpotassium carbonate (133 mmol/L, 0.3 mL) and a solution of 40 mg ofKryptfix 222 (under trade name, manufactured by Merck & Co., Inc.) in1.5 mL of acetonitrile was allowed to pass through the same column toelute [¹⁸F]fluoride ion.

The eluate was heated to 110° C. to evaporate water, and was subjectedto azeotropic distillation with addition of acetonitrile (0.5 mL×2),followed by evaporation to dryness. To the dried [¹⁸F]fluoride, asolution of 32 mg ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ethyl ester obtained in Example 1 in 1 mL of acetonitrile was addedand heated at 85° C. for 3 minutes. Then, 4 mL of diethyl ether wasadded to the solution and further 3 mL of the same was added twice, andthe mixture was allowed to pass through Sep-PakSilica (under trade name,manufactured by Japan Waters) to yield a solution of a[¹⁸F]fluorine-labeled compound in acetonitrile/diethyl ether.

To the obtained solution of the [¹⁸F]fluorine-labeled compound inacetonitrile/diethyl ether, 1.5 mL of 4 mol/L hydrochloric acid wasadded and heated at 120° C. for 15 minutes to perform deprotection toyield anti-[¹⁸F]FACBC. The obtained anti-[¹⁸F]FACBC was subjected to gaschromatography to quantitatively determine methanol and ethanol. Asshown in Table 2, no methanol was detected, while ethanol was detectedat concentrations of 24.1±0.8 ppm.

The results so far confirmed that the use of a compound according to thepresent invention as a labeling precursor makes it possible to preventmethanol from remaining in the synthesized anti-[¹⁸F]FACBC.

TABLE 2 Analyses of methanol and ethanol Average Standard SolventContent (ppm) (ppm) deviation Methanol 1 not detected 2 not detected 3not detected Ethanol 1 24.5 24.1 0.8 2 23.1 3 24.6

INDUSTRIAL APPLICABILITY

The compound of the present invention provides radioactivehalogen-labeled organic compounds which are used as radiopharmaceuticalsin nuclear medicine examination using PET or SPECT, and is useful in thefield of radiopharmaceuticals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scheme of synthesis ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-benzyloxy-cyclobutane-1-carboxylicacid ethyl ester;

FIG. 2 shows a scheme of synthesis ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-hydroxy-cyclobutane-1-carboxylicacid ethyl ester; and

FIG. 3 shows a scheme of synthesis ofsyn-1-(N-(t-butoxycarbonyl)amino)-3-[((trifluoromethyl)sulfonyl)oxy]-cyclobutane-1-carboxylicacid ester.

The invention claimed is:
 1. A method for the production of1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid from a precursorcompound of formula (2):

wherein the precursor compound of formula (2) produces ethanol ratherthan methanol during the production of1-amino-3-[¹⁸F]fluorocyclobutanecarboxylic acid from the precursorcompound of formula (2), wherein the method includes adding radioactivefluoride to the precursor compound, and wherein the method comprisesreacting a compound of formula (7):

with trifluoromethanesulfonic anhydride in pyridine.
 2. The methodaccording to claim 1, wherein the reaction of the compound of formula(7) is conducted using 1.5 molar equivalents of trifluoromethanesulfonicanhydride.
 3. The method according to claim 1, wherein the reaction iscarried out using 3.25 mL pyridine/mmol of the compound of formula (7).4. The method according to claim 1, wherein the reaction is conducted at0° C.
 5. The method according to claim 1, wherein the compound offormula (7) is obtained from a compound of formula (6) by removal of thebenzyl group


6. The method according to claim 5, wherein the compound of formula (6)is obtained from a compound of formula (5) through Boc protection of theamine


7. The method according to claim 6, wherein the compound of formula (5)is obtained from a compound of formula (4) by ethyl esterification ofthe carboxylic acid


8. The method according to claim 7, wherein the compound of formula (2)is prepared using the compounds of formula (4)-(7) in the followingorder:


9. The method according to claim 8, wherein the compound of formula (2)is prepared according to the following scheme:


10. The method according to claim 1, wherein the compound of formula (7)is dissolved in pyridine to which the trifluoromethanesulfonic anhydrideis thereafter added.